The Steelers won a tough battle in a hard-fought and fortunate, 23-22, win at Miami on Sunday. But they may well have lost the war of the 2010 season.

Aaron Smith, the Steelers’ best defensive lineman and arguably the NFL’s top 3-4 defensive end over the past decade, left the game early in the second half with what is believed to be a torn triceps. The injury will likely end his season, per the Pittsburgh Post-Gazette.

This will mark the third time in the last four years that the now-34-year-old Smith suffered a career-ending injury during the season. In the other two seasons (2007, 2009), the normally stout Pittsburgh defense was nowhere near as formidable without Smith and essentially collapsed down the stretch of both seasons.

In between, Smith stayed healthy for the 2008 season, earning a Pro Bowl selection and winning a second Super Bowl title with the Steelers. His play thus far this fall was similar to much of his career when healthy.

The Steelers will now likely move reserve Ziggy Hood into a starting role at LDE. Hood, however, has been bothered by a sprained ankle this fall and has yet to really flash the ability in games that made him the Steelers’ first-round pick in 2009. Pittsburgh desperately needs a quick return to health by veteran starting RDE Brett Keisel, who missed today’s Miami game with a pulled hamstring.

The Pittsburgh defensive line provided almost no pass rush and was not as strong against the run with Hood and journeyman reserve Nick Eason at end today throughout much of the second half. Players behind them engender even less confidence.

Second-year player Steve McLendon has been going back and forth between the active roster and practice squad this fall. Expect him to now be signed to the active roster for the rest of the season as soon as Smith is placed on injured reserve.

If Keisel is not back for the Sunday night showdown at New Orleans next week, Pittsburgh may also need to sign former LSU star Al Woods off its practice squad just to have a two-deep roster on the defensive line, because the team only kept six on its original 53-man roster, with five of those players over 30.

Pittsburgh then would have to temporarily release a player from another position (likely QB Charlie Batch or QB Byron Leftwich) to make room on the roster for Woods for a game or two before resigning the released quarterback.

Regardless of any roster moves, expect reserve nose tackle Chris Hoke to also start receiving more game reps at defensive end as well. In some seasons, Hoke has not played any defensive end. However, he has already seen a few reps on the outside this season and will now likely have to cross-train at NT and DE, where the Steelers’ depth will lack experience and talent.

Smith is signed through 2011 and the Steelers would likely welcome him back next season, but it may have to be as a part-time player and at a lower salary. However, he may just decide to retire, thus ending a fine career. Even if Smith returns next fall, the Steelers can no longer depend on him playing 16 games.